Known from the prior art are different kinds of dryers for use in different kinds of processes. In addition, different kinds of dryers based on a fluidized bed and circulating mass technology are known. In terms of this invention, the closest prior art is represented by the fluidized bed and circulating mass dryers that recuperatively use hot water or water vapor as the heat source.
At the bottom of fluidized bed dryers, the vertical speed of the fluidization material is zero and the volume fraction of solid material typically ranges from 0.2 to 0.5. In the space above the fluidized bed, the volume fraction of solid material is typically <0.001, in which case the fluidization material flow exiting the dryer is small. In circulating mass dryers, the volume fraction of solid material in the fluidized bed typically ranges from 0.1 to 0.3 and in the area above the fluidized bed, such as in a riser, from 0.005 to 0.05. Due to the high solid material content in the riser, the circulating mass dryers are provided with a separator and a return channel, so that the solid material that exits the riser can be returned back to the fluidized bed.
In fluidized bed dryers based on recuperative heat delivery, the heat delivery surfaces must be fitted to the thick fluidized bed and so are subject to heavy wearing caused by the fluidization material. To prevent obstructions, the heat delivery surfaces must be packed loosely. To be able to fit a required amount of the heat delivery surface to the fluidized bed, its volume becomes large. For these reasons, the internal consumption of bubbling fluidized bed dryers is high. In addition, the heat delivery surfaces fitted to the fluidized bed impair the mixing of solid material in the fluidized bed and, especially when sludges are being dried, the risk of fouling and obstruction of the heat delivery surfaces is high. The problems of bubbling fluidized bed dryers also include a risk of fire and explosion, because the volume fraction of solid material above the fluidized bed is small, allowing the dry dust to form, in the presence of oxygen, an explosive mixture. Due to heavy wearing, a heat exchanger fitted to the fluidized bed requires a great deal of servicing, which limits the usability of the dryer.
To remedy the above-mentioned defects of bubbling fluidized bed dryers, circulating mass dryers in which the heat delivery surfaces are fitted above the fluidized bed have been developed. The first circulating mass dryer based on recuperative heat delivery is disclosed in Finnish patent FI 105853. The invention is characterized in that the riser of the circulating mass dryer fitted above the fluidized bed having the shape of a circular cylinder at the bottom of the dryer is formed by the tubes of a vertical tube heat exchanger having the shape of a circular cylinder, and that heat releasing water or steam is conveyed to the jacket side of that heat exchanger. In the invention of patent FI 105853, the rotationally symmetrical, multiple-opening cyclone of the circulating mass dryer is coaxially fitted above the heat exchanger in such a way that the tube located in the middle of the heat exchanger forms a return channel for the circulating mass. The dryer of patent FI 105853 and the circulating mass dryers based on recuperative heat delivery formed later on that basis remedy the above-mentioned defects of fluidized bed dryers, but not the problems that inevitably relate to the recuperative use of water or steam, the most significant ones being the expensive pressurized hot water or steam system, expensive pressurized structure and high internal consumption. The price of the dryer is further considerably raised if, for the production of hot water or steam, a separate boiler unit must be built for the dryer.